In most countries they are not. It's really only a big problem in the US, where portion sizes are absurdly large and fast food is cheaper than anything else.
This actually isn't remotely true. It's far cheaper to buy more nutritious foods from the grocery store (especially in bulk) and then cook your own meals: http://www.leannebrown.ca/good-and-cheap.pdf
You can't have organic beef every night, but you can nutritionally and calorically out-gun fast food for the same (or a smaller) amount of money.
But if I'm poor, the difference between a ten-pound bag of potatoes and one fast-food meal can really matter.
A also believe that healthy meals can potentially increase my lifespan and quality of life within that lifespan. (I don't know if science backs this up). When I cook for myself I'm assured that cheap by unhealthy foods are not hidden in my meals - even the most expensive restaurants won't do that. (Trans fats and excess salt are two examples)
> Reasons for this issue include mining operations that have left not much arable land; as a result, much of the local diet is of processed, calorie-dense, imported food such as Spam or corned beef, rather than fresh fish, fruit and vegetables